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  • 2013 J o h n s h o p k i n s U n i v e r s i t y

    Physics & Astronomy

  • J O H N S H O P K I N S U N I V E R S I T Y P H Y S I C S A N D A S T R O N O M Y 2 0 1 3 1

    Letter from the Chair

    Dear alumni, colleagues, and friends,

    I am pleased to be writing to you at the end of another exciting year here in the Department of Physics and Astronomy at Johns Hopkins. Many impressive developments have been taking place across the full spectrum of our activities.For starters, I want to draw your attention to a once-in-a-lifetime event: the upcoming

    release of our faculty member David Kaplans movie Particle Fever, which tells the story

    of the Large Hadron Collider (LHC) and the discovery of the Higgs boson. I saw

    Particle Fever at the New York Film Festival, and I can tell you that it is a winner!

    It opens nationwide in March 2014.

    We continue to make great strides in research in the department, from Oleg

    Tchernyshyovs beautiful and elegant work on quantum spin systems, to Alex Szalays

    leadership in the science of big data, to the critical role played by Andrei Gritsan and his

    team in elucidating the properties of the Higgs boson at the LHC. We are also pleased that

    our researchers continue to garner significant awards and recognitionfrom Holland Ford

    receiving NASAs Distinguished Service Medal for his key contributions to the Hubble Space

    Telescope, to graduate students Schuyler Wolff and Alex Greenbaum, who were awarded NSF

    fellowships, and first-year graduate student Guy Marcus winning the APSs Apker Award.

    I also want to draw your attention to the new version of our General Physics course that

    takes place in a beautifully renovated Bloomberg 478 (see pages 12-13). Nearly 60 percent

    of Hopkins students take General Physicsthats almost 700 students per semesterand we

    are working across the board to improve our teaching in these classes that are so critical to

    our mission. After a set of visits by department members to assess teaching practices at other

    universities, we have also developed a new active-learning-based curriculum for our discussion

    sections that are led by our teaching assistants.

    Finally, be sure to check out our brand new physics and astronomy website at The new site allows for much easier navigation and offers

    an overview of all that is happening in the department.

    Enjoy this issue, and thank you for your interest in and support of physics and

    astronomy at Johns Hopkins.

    Best,Daniel Reich, Chair

    The Henry A. Rowland Department of Physics and Astronomy


    Letter from the Chair 1

    Tracking Down the Centerpiece

    of Particle Physics 2

    The Next Scientific Revolution: Big Data 4

    The Mysteries of Magnetism 6

    News Briefs 8

    Physics and Astronomy is an annual publication of the Johns Hopkins University Zanvyl Krieger School of Arts and Sciences Department of Physics and Astronomy. Send correspondence to: Kate Pipkin, 3400 N. Charles Street, Wyman 500W, Baltimore, MD 21218 or

    EditorKate Pipkin

    DesignerKathryn Vitarelli

    WritersLatarsha GatlinGabriel PopkinTom SiegfriedJoe Sugarman

    PhotographyWill Kirk, Homewood Photography (unless otherwise noted)

    On the cover: The Bloomberg

    Center is home to a new data

    center that is quickly becoming

    a hub for the universitys data-

    intensive computing efforts. For

    the cover, our photographer

    shot a server cluster using a

    zoom lens and a slow shutter

    speed, to create the illusion of

    the computer lights moving.

    J o h n s h o p k i n s U n i v e r s i t y

    Physics & Astronomy

  • 2 J O H N S H O P K I N S U N I V E R S I T Y P H Y S I C S A N D A S T R O N O M Y 2 0 1 3 J O H N S H O P K I N S U N I V E R S I T Y P H Y S I C S A N D A S T R O N O M Y 2 0 1 3 3

    O n June 14, 2012, Associate Professor Andrei Gritsan, postdoctoral fellow Sara Bolognesi, and graduate student Andrew Whitbeckall from Johns Hopkinsgathered with several colleagues for a secret night-time meeting in a conference room at the CERN laboratory in Geneva, Switzerland. The occasion was their first look at a statistical analysis of data from proton collisions in the Large Hadron Collider (LHC), the worlds largest particle accelerator. So important were the data that the researchers had analyzed them without actually looking at the results, to avoid any chance of biasing their conclusions.

    At this point, the researchers knew a graph with a bump in it would indi-cate strong evidence the collider had produced the elusive Higgs boson. The Higgs, first proposed in the 1960s, was needed to fill in the largest gap in the

    Standard Model, the leading theory in fundamental particle physics. Its exis-tence would signify the existence of a field that permeates all space and imbues the matter in the universe with mass.

    A flat graph, by contrast, would meannobody was quite sure what.

    Gritsan, the leader of the team working at CERN, projected the graph on a screen, and the scientists knew their long wait was over.

    It all changed in an instant, says Gritsan. It was an emotional moment, and it left no doubt that we had something big.

    On the strength of the two bumps (another one revealed in another chan-nel) and similar evidence from another research group, CERNs leaders called a press conference for July 4, 2012. At the conference, they announced to the world that they had found a new particle with a mass of around 125 billion electron-

    volts, or GeVs (the electronvolt is particle physicists preferred way to express mass; 1 eV indicates a mass of about 1.8 10-33 grams). Whitbeck was delegated to sit near the front of the room, armed with a battery of PowerPoint slides to ex-plain the scientists analysis techniques, should any reporter ask. (None did.)

    Left unanswered at the conference, how-ever, was whether the new particle was ac-tually the Higgs boson. At the time, LHC researchers were even reluctant to call the particle a Higgs boson, preferring instead the cautious Higgs-like particle. Now, a year and a half later, LHC physicists are much more comfortable saying they have found a Higgs, if not yet the Higgs. And much of that progress has been due to efforts of the Gritsan lab at Johns Hopkins.

    Gritsan and his team are part of the more than 2,000-person collaboration that manages the Compact Muon Solenoid, or CMS, one of two enormous detectors

    Tracking Down the Centerpiece of Particle PhysicsJHU Physicist Plays Key Role in Finding Elusive Particle By G a B r i e l p o p k i n

    nestled into the LHCs 17-mile-long tunnel. Using arrays of sensors similar to those in a digital camera, the detector takes precision snapshots of the debris spewed out when near-light-speed protons collide and turn their energy into new matter. The CMS was already under construction when Gritsan joined the collaboration in 2005, but he quickly made his mark on the team, finding ways to precisely align the instruments sensors so researchers could compute the exact paths particles would take through the detector.

    Gritsan simultaneously developed meth-ods to extract useful results from the masses of data the CMS would soon be collecting. The CMS, along with its sister detector ATLAS, was designed to capture not the Higgs boson itself but its decay products, because theorists had shown that the Higgs boson, if it exists, will decay into other particles before it can leave any physical record of its presence. Gritsan and his team

    looked for signatures of one of the Higgs bosons possible decay channels, known as HgZZ. In this channel, the Higgs decays to two Z bosons; these in turn quickly decay to four leptonsa class of particle that includes electrons and their heavier cousins, muons. As the leptons speed away at close to the speed of light, they leave traces in the layers of sensors that make up the CMS. The sensors then dump their data to a worldwide network of computers.

    Thats when the analysis started. The amount of data to be sifted through was truly daunting20 billion collisions recorded, each yielding roughly 1 mega-byteand the researchers only hope was to write clever computer programs to extract signals of the particles they sought from this vast, noisy background. Gritsan, Bolognesi, Whitbeck, and several former students in the lab spent years developing a sophisti-cated analysis method called the Matrix Element Likelihood Approach, or MELA

    (mela means gathering in Sanskrit and apple in Italian; Bolognesi says she and her colleagues hope their technique is as successful as Apple Computers). MELA, which extracts information on the angles at which decay particles fly away from a collision, is instrumental in amplifying the signal to the so-called five-sigma certainty level that gave LHC leaders the confidence to call the famous 2012 press conference. In early 2013, MELA results gave physicists the confidence to call it a Higgs boson, and in October, the two researchers (Peter Higgs and Francois Englert) who first predicted its existence received the Nobel Prize.

    With such blockbuster success, LHC scientists now find themselves in a pecu-liar position. On the one hand, they have made the biggest discovery in particle physics in decades. On the other hand, it is a discovery many people expected; and not finding the Higgs boson would have in some ways been more tantaliz-ing. Meanwhile, the LHC is shut down until 2015 for repairs and upgrades, so any new revolutionary discoveries are at best several years down the road.

    But Gritsan and his